Power switching devices such as power MOSFETs are requested to be controlled so as to be normally off in order to ensure safety during operation. Compared to silicon MOSFETs, next-generation devices such as SiC (silicon carbide)-J (junction) FET and GaN (gallium nitride) FET can greatly reduce losses and enable normally-off operation. However, many of these FETs accept up to several voltages (approximately 2 V to 3 V) while an Si power MOSFET can accept a voltage of 10 V to 20 V to the gate. Such devices cannot be driven at a high voltage and therefore make high-speed switching difficult.
As technologies of limiting gate voltages, for example, JP-A-3655049 (see FIG. 11) uses a series circuit of a gate resistor and a zener diode connected between the emitter of a gate drive transistor and the ground. The gate of a static induction transistor is connected to a common connection point of both, allowing a zener voltage to clamp a gate voltage. This configuration allows an electric current to continuously flow through the zener diode while the static induction transistor is turned on. In this configuration, the drive circuit is subject to a large loss and is incapable of switching in a high frequency range.
JP-A-4321330 (see FIG. 14) uses four switching devices to configure a gate drive circuit similar to an H bridge. The bridge circuit allows an inductor to flow a gate current. This enables to fast switch the MOSFET independently of a gate voltage and decrease a loss occurring in the drive circuit.
However, the configuration according to JP-A-4321330 requires a long time to increase a current applied to the inductor and is also incapable of switching in a high frequency range.